During the winter of 1971/72, eleven dermatologists in private practice and the dermatological departments of the Finsen Institute and Marselisborg Hospital conducted a controlled clinical trial to investigate the effect of topically applied tretinoin in patients with acne vulgaris using a 0.02-percent cream, a 0.05-percent cream, and the cream base alone. The effect of the treatment was evaluated by counting the acne lesions, the number being arranged according to a score system. As regards the effect on comedones and papules, the tretinoin cream was significantly better than the cream base alone. A concentration of 0.05% showed a quicker onset of action and a more pronounced effect than a concentration of 0.02%. The effect of the treatment was less pronounced and not statistically significant as far as pustules and cysts were concerned. The investigation confirmed previous good experience with this form of treatment. The effect upon the placebo group was considerable. This calls for caution when the effect of new acne agents is to be evaluated. Attention is drawn to the special form of cooperation used in the investigation; in this way, it was possible to include a great number of out-patients.